The RAN Minehunter-Inshore vessels HMAS Rushcutter and HMAS shoalwater have experienced paint blistering on immersed areas of the hulls of both ships. The hull surfaces are glass reinforced polyester (HRP) resin in a GRP-foam-GRP sandwich construction. Paint blistering has occurred from the inner hull surface in integral fuel and water tanks as well as from underwater areas on the outer hull surface. In view of the apparent degradation of the polyester in the tanks following coating failure, an investigation was carried out to determine the cause of the adhesion loss. Infrared examination of the failed areas showed the cause of paint detachment to be a poorly cured polyester layer under the paint. This layer softened on immersion in water of hydrocarbon fuel to the extent that it is readily suffered mechanical damage. Possible causes for the lack of cure of the surface polyester are discussed. Methods of treating the GRP to remove the uncrosslinked polyester layer were examined including sanding, sandblasting and chemical cleaning. Sandblasting was found to be the most effective surface treatment and significantly improved the adhesion of all paint coatings. Examination was also carried out to evaluate the effectiveness of alternative paint schemes including epoxy polyamide, epoxy polyamine (solventless), epoxy polyurethane and polyurethane formulations. The results showed that the polyurethane paints had excellent adhesion and were generally more tolerant of poor surface condition than solvent-borne epoxy paints.